Superabsorbent polymers (SAPs) or hydrogels or hydrocolloids are capable of absorbing many times their own weight of fluids such as water and retain it under moderate pressure. These materials are of diverse chemical origins as recognized in the prior art. Owing to their ability to absorb fluids, they find extensive use in sanitary products including baby napkins, meant to absorb baby urine and faecal moisture; female sanitary pads meant to absorb menstrual fluid and others. Superabsorbent polymers absoro more than 80 g of water per gram of the xerogel (dry polymer), unlike the common absorbent materials (Table 1).
TABLE 1Water absorbency of some common absorbent materials in comparisonwith an indigenous SAP sampleAbsorbent materialWater absorbency (%)Whatman No. 3 filter paper180Facial tissue paper400Soft polyurethane sponge1050Cotton Ball1200Pusa Hydrogel>35,000
Water absorption capacity (WAC) is the most important characteristic of the superabsorbent polymers. There are many ways to measure WAC, though there is no accepted standard yet. Usually, it is measured using volumetric or gravimetric or spectroscopic or microwave method. The volumetric method measures the volume change of SAP (or the water) before and after the absorption, the gravimetric method measures the weight change of SAP, the spectrometric method measures the change in the UV-spectrum of the SAP and the microwave method measures the microwave absorption by energy changes.
The WAC of a SAP depends upon its composition and structure generated by the preparation method, as well as the presence of electrolytes in water. For example, the WAC of SAP can be several thousand grams water per gram SAP when in contact with pure water, but in water containing urine, blood, metal ions etc., it will be reduced by several folds the maximum value in pure water. Water absorbed in the SAP can exist as bound water, half-bound water and free water. Free water shows a freezing point when the environment temperature touches around 0° C., however, this freezing point cannot be noticed with the bound water. The half-bound water shows property in between. The bound water in SAP usually is 0.39-1.18 g per g. Most water in the SAP is free water. Tatsumi studied the effect of chemical structure on the amount of microwave absorption of water in various polymer films at 9.3 GHz. The absorption was directly proportional to both the volume increase of the sample film and the amount of water in the polymer. The principle of water absorption by polymer can be illustrated by the Flory theory of an ionic network.Q5/3={(1/2×I/Vu×1/S1/2)+(1/2X1)/V1}×V0/ν
Where Q=maximum swelling ratio of SAP, I=electronic charge on the polymer structure per polymer unit, Vu: polymer repeating unit volume, S=ionic strength of solution, X1=interaction parameter of polymer with solvent, V1=molar volume of solvent, in a real network, V0=un-swollen polymer volume, ν=effective number of chains. These parameters in the equation formed a balance of the swelling which can be further defined as follows: 1/2×I/Vu×1/S1/2 denotes ionic strength on both polymer structure and in the solution, (1/2·X1)/V1 denotes the affinity of network with solvent; V0/ν is cross-linking density. The equation shows that the water absorption is a function of osmotic pressure, the affinity of water and polymer, and the cross-linking density of the network.
The swelling process of SAP can be explained as follows: the solvent tries to penetrate the polymer networks and produce a 3D-molecular network, expanding at the same time, the molecule chain between the crosslinked points, thus decreasing the configuration enthalpy. The molecular network has an elastic contractive force which tries to make the networks contract. When these opposed forces reach equilibrium, the expansion and contraction reach a balance too. In this process, the osmotic pressure is the driving force for the expansion of swelling, and the network elastic force is the driving force of the contraction of the gel.
These materials find important application in agriculture as water retaining soil conditioners, a use that is likely to catch up as water is recognized to become the most valuable and scarce commodity in future. The technologies and products that conserve and promote its judicious and efficient use are likely to be sought after in the future.
The term soil conditioner implies compounds, which favourably alter the physical and/or chemical properties of soil. The concept of using polymer materials as soil conditioners is not new. Natural polymers such as polyuronic acids, alginic acids, agar, gum, pectin, starch, etc. have been successfully used in the past for soil conditioning. However, their easy biodegradation and low water holding property are bottlenecks in practical use.
Purely synthetic SAPs include polyacrylates, sulfonated polystyrene, polyvinyl alcohol, polyethylene oxides, polyvinylpyrollidone, polyacrylonitriles, polyacrylamide and the like. Some of these like polyacrylamide have been used for water retention purposes in agriculture.
Scale of performance and economic considerations have evoked interest in the development of SAPs and SAP composites. Superabsorbent composites based on natural raw materials and clay minerals, which can be chemically entangled with hydrophilic units of synthetic superabsorbent polymers to yield products with superior water holding characteristics and the desired persistence will go a long way in improving their application prospects in future agriculture. A need also exists for a superabsorbent material which combines the advantage of liquid absorption potential of conventional SAPs, advantageous liquid distribution properties of biopolymer and permeability and mechanical stability of clay minerals, by virtue of which the resultant hydrogels do not form soft gelatinous masses when hydrated, have good absorbent properties, gradual releasing potential and controlled biodegradability. Moreover, there is a need for a simple, convenient and inexpensive method for making such materials, the aspects that have been explored in this invention
Variety of superabsorbent polymers have been developed following different procedures and used under diverse use situations. U.S. Pat. No. 3,669,103 discloses a process for acrylic acid and acrylamide based gelling polymers for use in personal care products. U.S. Pat. No. 6,500,947 describes a method of making superabsorbent hydrogel from cellulose fibres obtained from wood pulp, by sulfonation of the fibres. However, the use of sulfuric acid renders the hydrogel mechanically unstable resulting in soft gelatinous mass on exposure to water, making it difficult to handle in practice.
In U.S. Pat. No. 4,244,880, hydrogels meant for temperature controlled solute delivery system in human body include crosslinked poly N-isopropylacrylamide and crosslinked cellulose ether gels. The method used involves exposure of the reaction mixture to nitrogen atmosphere containing less than 2% oxygen. Another example of such hydrogels is provided in U.S. Pat. No. 5,064,653, which describes hydrophilic foam compositions containing hydrogels belonging to the category of starch grafted on copolymers of acrylamide salts, acrylate salts and mixtures thereof. Hydrophilic properties of carboxymethyl cellulose have been utilized in the U.S. Pat. No. 3,586,648 in treatment of polyurethane foams in such a manner as to render the latter hydrophilic.
Use of crosslinked polyacrylamides in plant growing media is well established. U.S. Pat. No. 4,579,578 describes free radical polymerization of acrylamide in the presence of N, N-methylene bisacrylamide resulting in a hydrogel capable of absorbing 30 times its own volume of water.
Similar plant growth compositions are described in U.S. Pat. No. 4,559,074 wherein crosslinked non-ionic polyacrylamide has been incorporated into the porous growth medium. Yet another evidence of the versatile potential of carboxymethyl cellulose is provided by U.S. Pat. No. 6,387,978 reporting preparation of crosslinked carboxymethyl cellulose involving ionic crosslinking by anions or metal cations, non-ionic crosslinking by chemical crosslinking agent or high energy gamma radiations.
Similar type of polyacrylamide absorbent materials are known in the art and are described in U.S. Pat. Nos. 4,102,340; 3,229,769; 3,670,731 and Indian Patent Application No. 3462/DEL/2005. The polymerization techniques for the aforesaid materials include the use of anionic peroxide catalysts, photopolymerization with a riboflavin activator and the like.
Clay-polymer composites are also attracting ever increasing attention in recent years (Theng, 1974). Traditionally, clays have been used as fillers for improving a material's physico-chemical properties and reducing product cost. In 1985, an inorganic-organic composite (Superabsorbent Polymer Clay Composite, SAPC) was prepared by intercalating acrylamide into an expandable smectitic clay, such as bentonite using γ-ray radiation-induced polymerization (Rong et al, 1985). The preparation technique was improved and some of the properties of the composite material reported (Gao, 1993). The new material showed good absorption capacity to water and its vapours. The material also showed an interesting physico-chemical and electromechanical reaction to environmental changes such as temperature, moisture, electric fields, concentration changes of chemical species, and pH (Gao and Heimann, 1993) and has been used in oil fields for enhanced oil recovery processes and in other areas such as agriculture, forestry etc.
When layered clays are filled into a polymer matrix, either conventional composite or nanocomposite is formed depending on the nature of the components and processing conditions. Conventional composite is obtained if the polymer can not intercalate into the galleries of clay minerals. The properties of such composites are similar to that of polymer composites reinforced by micro particles. As shown in FIG. 1, two types of nanostructures result from the mixing of clay minerals and a polymer depending on the reaction conditions. One is the intercalated nanocomposite (I), in which monolayer of extended polymer chains is inserted into the gallery of clay mineral resulting in a well ordered multilayer morphology stacking alternately the polymer layers and clay platelets with a repeating distance of a few manometers. The other is exfoliated or delaminated nanocomposite (II), in which the clay platelets are completely and uniformly dispersed in a continuous polymer matrix. In most cases, a cluster (the so-called partially exfoliated) nanocomposite (III) is common in polymer nanocomposites.
Of late, biopolymers are receiving growing interest due to environmental concerns, increasing prices of crude oil and global warming. These polymers are naturally occurring, being of plant or animal origin. Numerous biopolymers such as starch, chitosan, derivatives of alginic acid, carrageenan, polylactic acid, cellulose and its derivatives, rubber etc. have been exploited to synthesize biopolymer based composites/nanocomposites (Chang et al, 2003; Mathew et al, 2005; Wu et al, 2004; Kvien et al, 2005; Mathew et al, 2006; Ray et al, 2003; Carvalho et al, 2001; McGlashan and Halley, 2003; Park et al, 2003; Wang et al, 2005; Xu et al, 2005; Pourjavidi and Mandavivia, 2006; Pourjavidi et al, 2007).
Synthesis of superabsorbent hydrogels using conventional heating is increasingly being replaced by greener techniques such as microwave assisted polymerization (Singh et al, 2004; Singh et al, 2005; Duan et al, 2008). However, such techniques have not been tried as yet in the synthesis of superabsorbent composites.